For pull requests, get the source branch name from github

This python script does the right thing testing on my computer.  I haven't tested it on travis, for obvious reasons.  For example, if travis python is actually python 3, it won't work.

Following @byorgey's suggestion on IRC:
```
<byorgey> so, our travis stuff does the right thing when building branch X
          that depends on branch X from another repo (it looks for a branch of
          the same name)  [22:12]
<byorgey> but it looks like it doesn't do the right thing for pull requests
<byorgey> because when building a pull request it thinks it is building the
          branch being merged into rather than the branch being merged from
                                                                        [22:13]
<byorgey> so it checks out the wrong branch of the dependencies.
<fryguybob> byorgey: Ah, bummer.  [22:17]
<bergey> byorgey: Yes, I remember running into that with the lens branches.
<byorgey> we can find out from travis if we are building a pull request (with
          the TRAVIS_PULL_REQUEST env var)  [22:18]
<byorgey> then we could use the github API to find out the name of the branch
          the PR originated from.
<byorgey> seems like a lot of work, but this probably won't be the last time
          this situation comes up  [22:19]
```