resolve #153: git ship errors with empty commit message

@kevgo 

Can't think of a way to test this with our suite. 
Passing in an empty message `git ship -m ''` currently still opens the editor.
Until we find a way to interact with the editor unsure what to do.
Tested it by hand (after commenting out push_branch and delete_branch, lines 100-101, from git-ship, just to be safe).

```bash
$ git ship

[cr-fixGitShipCommitAbort] git checkout master
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.

[master] git fetch --prune

[master] git rebase origin/master
Current branch master is up to date.

[master] git checkout cr-fixGitShipCommitAbort
Switched to branch 'cr-fixGitShipCommitAbort'

[cr-fixGitShipCommitAbort] git merge --no-edit master
Already up-to-date.

[cr-fixGitShipCommitAbort] git checkout master
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.

[master] git merge --squash cr-fixGitShipCommitAbort
Updating 904a837..15f6969
Fast-forward
Squash commit -- not updating HEAD
 git-ship               | 17 +++++++++++++++--
 helpers/git_helpers.sh |  7 +++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

[master] git commit -a
Aborting commit due to empty commit message.

[master] git reset --hard
HEAD is now at 904a837 Better formatted release notes

[master] git checkout cr-fixGitShipCommitAbort
Switched to branch 'cr-fixGitShipCommitAbort'

  Error
  Aborting ship due to empty commit message

```