planemo ci_find_repos - $TRAVIS_COMMIT_RANGE Problem executing commands

This test run was from a rebased branch which had been force updated, meaning TravisCI does not have a meaningful `$TRAVIS_COMMIT_RANGE` thus as per https://github.com/galaxyproject/tools-iuc/pull/973 and https://github.com/galaxyproject/tools-iuc/issues/961 the `.travis.yml` file set this to differences from the master branch.

Unfortunately `planemo ci_find_repos` then breaks, e.g. https://travis-ci.org/peterjc/tools-iuc/jobs/165764988

```
$ if ! git diff --quiet "$TRAVIS_COMMIT_RANGE" -- ; then TRAVIS_COMMIT_RANGE=master...; fi
fatal: bad revision '13f515d37121...2ebff2330354'

$ echo $TRAVIS_COMMIT_RANGE
master...

4.63s$ planemo ci_find_repos --exclude_from .tt_blacklist \
--exclude packages --exclude data_managers \
--changed_in_commit_range "$TRAVIS_COMMIT_RANGE" \
--chunk_count 4 --chunk "${CHUNK}" \
--output changed_repositories_chunk.list

cd '/home/travis/build/peterjc/tools-iuc' && git diff --name-only 'master...' --
Traceback (most recent call last):
File "/home/travis/virtualenv/python2.7.9/bin/planemo", line 11, in
sys.exit(planemo())
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/core.py", line 716, in __call__
return self.main(*args, **kwargs)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/core.py", line 696, in main
rv = self.invoke(ctx)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/core.py", line 1060, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/core.py", line 889, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/core.py", line 534, in invoke
return callback(*args, **kwargs)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/decorators.py", line 64, in new_func
return ctx.invoke(f, obj, *args[1:], **kwargs)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/click/core.py", line 534, in invoke
return callback(*args, **kwargs)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/planemo/cli.py", line 175, in handle_profile_options
return f(*args, **kwds)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/planemo/commands/cmd_ci_find_repos.py", line 25, in cli
paths = filter_paths(ctx, raw_paths, path_type="dir", **kwds)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/planemo/ci.py", line 24, in filter_paths
diff_files = git.diff(ctx, cwd, changed_in_commit_range)
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/planemo/git.py", line 53, in diff
cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE
File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/planemo/io.py", line 43, in communicate
raise RuntimeError(msg)
RuntimeError: Problem executing commands cd '/home/travis/build/peterjc/tools-iuc' && git diff --name-only 'master...' -- - (, fatal: bad revision 'master...'
)
The command "planemo ci_find_repos --exclude_from .tt_blacklist \
--exclude packages --exclude data_managers \
--changed_in_commit_range "$TRAVIS_COMMIT_RANGE" \
--chunk_count 4 --chunk "${CHUNK}" \
--output changed_repositories_chunk.list
" failed and exited with 1 during .
Your build has been stopped.
```