Run clang-tidy on Travis with specific warnings as errors.

[iphydf]

1. Enable all the warnings.
2. Selectively disable warnings that are currently triggered.

This way we can ensure that we at least won't cause regressions in terms of warnings.

Then later:

3. When we're bored, go through one warning at a time and see if we can/should fix it, or if we should keep it disabled or silenced for a specific instance of the warning.

[shreyam19]

how to enable all the warnings?

[iphydf]

clang-tidy -checks=* -warnings-as-errors=some-specific-warning,other-warning,warning-class-*

[iphydf]

Done.