[BUILD] disable -Werror by default

- configure travis to use -Werror in gcc to check all incoming pull
  requests
- Werror does not seem to work on all configurations reliably (e.g.
  exclusion of system libraries) and changes from one compiler version
  to another make this not a safe option for an initial compile of
  OpenMS
Merge pull request #841 from OpenMS/feature/EnableWerror

[BUILD] disable -Werror by default