Adjust TRAVIS_COMMIT_RANGE variable to contain only commits from a PR.

[igchor]

Ref: travis-ci/travis-ci/issues/4596

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This change is 

[pmem-bot]

@kkajrewicz and @hubertsoltysiak please review this pull request

[codecov]

Codecov Report

Merging #265 into stable-1.5 will decrease coverage by 5.53%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           stable-1.5     #265      +/-   ##
==============================================
- Coverage       96.29%   90.76%   -5.54%     
==============================================
  Files              31       26       -5     
  Lines            2537     1050    -1487     
==============================================
- Hits             2443      953    -1490     
- Misses             94       97       +3

Flag	Coverage Δ
#tests_clang_debug_cpp17	91.44% <ø> (-5.37%)	⬇️
#tests_clang_release	90.31% <ø> (-5.84%)	⬇️
#tests_gcc_debug	93.39% <ø> (-4.03%)	⬇️
#tests_gcc_release_cpp17	89.39% <ø> (-6.15%)	⬇️
#tests_gcc_release_cpp17_no_valgrind	?

Impacted Files	Coverage Δ
include/libpmemobj++/shared_mutex.hpp	64.44% <0%> (-16.16%)	⬇️
include/libpmemobj++/detail/pexceptions.hpp	71.42% <0%> (-16.08%)	⬇️
include/libpmemobj++/mutex.hpp	65.51% <0%> (-14.49%)	⬇️
include/libpmemobj++/timed_mutex.hpp	72.09% <0%> (-9.73%)	⬇️
include/libpmemobj++/detail/common.hpp	83.33% <0%> (-7.37%)	⬇️
include/libpmemobj++/transaction.hpp	89.87% <0%> (-6.03%)	⬇️
include/libpmemobj++/detail/make_atomic_impl.hpp	94.44% <0%> (-5.56%)	⬇️
include/libpmemobj++/make_persistent_atomic.hpp	90.9% <0%> (-4.93%)	⬇️
...lude/libpmemobj++/make_persistent_array_atomic.hpp	95.23% <0%> (-4.77%)	⬇️
include/libpmemobj++/detail/life.hpp	94.11% <0%> (-4.48%)	⬇️
... and 15 more

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[szyrom]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved