Fix travis build to actually use gcc5

[tcm-marcel]

Travis vehemently ignores our environmental variables for gcc5, but setting it in front of the cmake command works.

@pmenon gave this hint before, but it was forgotten in #1311.

[pmenon]

@tcm-marcel For consistency, you should set CC=gcc-5, too.

Also, I wonder if you can export the CXX and CC variables before invoking the build? Then the cmake invocation should remain the same and, hopefully, make the environment setup clear.

[coveralls]

Coverage decreased (-77.9%) to 0.0% when pulling 29ba99e on tcm-marcel:build/deprecate-gcc48 into bf7ff62 on cmu-db:master.

[tcm-marcel]

@pmenon I just wanted to show you that this happens already when specifying it in the travis file.
But then I saw that in fact it does happen, but the variables get overridden a few lines later.

We can again override it just before the invocation.

[pmenon]

@tcm-marcel Huh, then it looks like the stuff in env is getting overridden. Does it make sense to move everything currently in env into before_script as export's?

I just found this as an example.

[tcm-marcel]

The tests fail because of an incompatibility between the pre-compiled tensorflow version and protobuf, which is already discussed on the mailing list. I will leave this open, until we have a solution/workaround for this.