Summary:
It looks like some proguard rename mapping files are of the form:
```
OldClsName -> ShortClsName:
   ...
```

This change fixes parsing these files. When the files don't have the trailing `:` this change is a no-op so it should be safe for any previous use cases.

Pull Request resolved: #514

Test Plan:
Filecheck tests call `dex.py`. Minimal soundness check...
`$ buck test $BUCK_MODES //fbandroid/native/redex/... --include redex_filecheck_test --exclude not_host_supported --always-exclude`
```
Testing: finished in 0.1 sec (20 PASS/0 FAIL)
```

Reviewed By: justinjhendrick

Differential Revision: D22817180

Pulled By: agampe

fbshipit-source-id: 86c44a51ba326c51695feb48ea29467d00a1d339