Travis config fix

[kulaginm]

• add 15beta1 to travis config
• fix travis warnings
  root: deprecated key sudo (The key sudo has no effect anymore.)
  root: missing dist, using the default xenial
  root: missing os, using the default linux
• fix gcc-7.5.0 preprocessor error
• fix clang scan-build warning

[codecov]

Codecov Report

Merging #109 (4813838) into master (7197b83) will decrease coverage by 6.68%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
- Coverage   42.05%   35.36%   -6.69%     
==========================================
  Files          32       35       +3     
  Lines       10768    13582    +2814     
==========================================
+ Hits         4528     4803     +275     
- Misses       6240     8779    +2539     

Impacted Files	Coverage Δ
src/tuplesort15.c	3.27% <0.00%> (ø)
src/rumsort.c	86.56% <0.00%> (-0.26%)	⬇️
pg_bin/include/postgresql/server/postgres.h	100.00% <0.00%> (ø)
pg_bin/include/postgresql/server/access/tableam.h	100.00% <0.00%> (ø)
..._bin/include/postgresql/server/utils/sortsupport.h	0.00% <0.00%> (ø)
src/tuplesort10.c	2.09% <0.00%> (ø)
...in/include/postgresql/server/access/htup_details.h	33.33% <0.00%> (ø)
src/btree_rum.c	96.79% <0.00%> (+0.01%)	⬆️
src/rumtsquery.c	90.15% <0.00%> (+0.03%)	⬆️
... and 12 more

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[pashkinelfe]

Committed 92f42d5 into master. Thanks!