Add ./node_modules/.bin to $PATH

[BanzaiMan]

travis-ci/travis-cookbooks#786 removed it, but
this broke many Ember-based repositories, so we will reinstate it.

This is meant to be a hot fix, which would be reverted after a new set
of build images are rolled out.

[backspace]

Someone suggested appending instead of prepending, would that be better to address the reason for this?

[BanzaiMan]

@backspace I don't think that'll work, because which is a Bash built-in function, and it's not on $PATH. That issue must be addressed separately.

[Turbo87]

@BanzaiMan right, that makes sense, thanks!