Update travis code format check

[StefanOberhumer]

Code format check like ethereum/evmc@b290ef3 but we're not exiting if format does not match clang-format style.

[chfast]

Is it useful in this form?

[StefanOberhumer]

Is it useful in this form?

I think so - we still do not generate an error if format differs.
Previous check did only warn the lines with trailing whitespaces and leading tabs.
As there is a .clang-format in the ethminer repo I think we should use it !

I came across when I was searching differences between my local clang-formatted version and the repo as I thought @jean-m-cyr clang formatted the whole (*.h, *.cpp) source. So I tried clang format in travis and found that I got the same diff as on my local version. I think all users should have the possibility to view the clang formatted output (even they have clang-format not installed)

Btw:

• clang-format is already installed in the travis image (so no extra installation is required).
• I'm thinking to test the git/hooks/pre-commit to automatically format my commits.

[chfast]

I'm ok with this change because it removes a script. But I'm rather sure contributors will not open Travis build page to check the formatting report.

This is good candidate for new circle CI job. I will work on this soon.

[StefanOberhumer]

... But I'm rather sure contributors will not open Travis build page to check the formatting report. ...

I know, but we can suggest to the travis result if we reformat the source.