Update handlebar-helpers

[dvera123]

handlebars-helpers current version is using an old version for minimatch, could you please update handlebars-helpers to version 0.7.3 ?

[assemblebot]

@dvera123 Thanks for the issue! If you're reporting a bug, please be sure to include:

• The version of assemble you are using.
• Your assemblefile.js (This can be in a gist)
• The commandline output. (Screenshot or gist is fine)
• What you expected to happen instead.