travis: fix bug in build/test script

The script was previously using `git show --pretty=format:%H` to print
out the commit hash of dependencies checked out of github. But also,
this command will still prints out the commit diff. For merge commits,
and non-merge commits whose diff isn't long enough for require the use
of a pager, this is not a problem. However, if the commit diff is long
enough that git pipes it to a pager, it will cause the build to hang.
Instead of using `git show`, the script now uses `git rev-parse HEAD`,
which will only print out the commit hash of HEAD.
Merge pull request #41 from frenetic-lang/travis

fix bug in build/test script