[travis] Fail if tabs found in CMake, cpp, c, h.
[travis] Apply @sherm1's stringent no-tabs policy.

More filetypes now need to obey no-tabs policy: .md .yml .cmake .xml .html .in
[travis] Test for tabs before all else.
[travis] no-tab check had redundant CMakeLists.txt
[travis] only perform no-tab check on linux.
Merge pull request #283 from chrisdembia/travis-fail

[travis] Fail if tabs found in CMake, cpp, c, h.