[build] Added clang format check to Travis

[maxsharabayko]

• Update Travis distro from trusty to xenial to support more recent cmake-format
• Added clang-format file
• Added a static syntax check of SRT core to Travis pipeline.

Clang Format Style Options

• Merge [build] Travis MinGW: updated OpenSSL to 1.1.1 #773 first.

Note: FixNamespaceComments: true requires clang-format version 5+.

Note

Format code:

 find ./srtcore/ -type f \( -name '*.cpp' -o -name '*.h' -o -name '*.hpp' -o -name '*.cpp' \) -print0 | xargs -r0 clang-format -i;

[ethouris]

It's too early. We need some less partial work to be done simultaneously.