Update xcode from 6.4 to 7.3.1

[Lnaden]

We should probably update the osx_image from xcode6.4 to at least xcode7.3 since it looks like Xcode 6.4 support may disappear from Travis any time (Supposedly they dropped support at the end of January)

It also looks like we can just drop the osx_image option from the travis file and it will just default to Xcode 7.3.1

Is there a reason we pin to a specific version of xcode other than for consistent build environment?

[Lnaden]

There was a typo on the Travis site, they retired xcode 6.1 on Jan 20th, no plans or timeline right now for xcode6.4. The typo has been fixed. (Summarized from Travis-CI support email I sent)

At some point we will need to deal with this, but for now I'm closing this issue.

[Lnaden]

I'd like to revive this issue with OpenMM 7.2 coming out soon and Travis moving to Xcode 8.3 as the default. This also means we will likely have to bump our min OSX version to 10.11, which would be a 2 year old OS instead of a 4 year old OS.

This would impact anyone deploying on OSX who deploys to a specific minimum version.

Something to think about.

[jchodera]

Tagging @peastman here.

The Travis blog post says that XCode 8.3 will be default starting Tuesday, 28 November 2017 (today).

However, we've already pinned to XCode 6.4, and this version is not slated for retirement, so this should not affect our ability to build OS X versions compatible with 10.9 and later at the moment.

After the release of OpenMM 7.2, however, we should rapidly explore the feasibility of migrating to conda-forge.

[peastman]

Is there any reason to change? It sounds like 6.4 is still supported and there's no difficulty in sticking with it. And I really don't like the idea of dropping support for OS versions that are only two years out of date.

[jchodera]

I don't see any reason to change the omnia xcode version at this time.

I'm not sure what a conda-forge version would support, however.

[Lnaden]

Conda forge uses CircleCI to do its OSX builds which supports Xcode 7.0, macOS 10.11 at the oldest

[darioizzo]

As a user, I support the change of XCode to a modern version at this point.

@peastman wrote:

Is there any reason to change? It sounds like 6.4 is still supported and there's no difficulty in sticking with it.

I think the reasons here are:

• The compilation speed of clang has improved significantly in recent XCode versions. I have cases dropping the build time from 45 to 15 minutes (package pagmo/pygmo on conda forge)

• Some C++ compilation may take more than 10 minutes in XCode6.4 halting the Travis CI and forcing ad hoc workarounds such as splitting files in multiple units which tend to lower the code quality.

[Lnaden]

Transferring relevant information from the duplicate issue

Travis support xcode 6.4 is going away in Jan 2019 (this month!):

Running builds with Xcode 6.4 in Travis CI is deprecated and will be removed in January 2019. If Xcode 6.4 is critical to your builds, please contact our support team at support@travis-ci.com to discuss options.

We'll need to update our builds to use a more modern xcode.