Remove GHC 7.10.3 travis build.
Remove GHC 7.10.3 travis build.

The -Wcompat flag is not recognized by GHC 7.10.3, but rather than
conditionally enable this flag, the 7.10 version is now over 3 years
old (released Dec 2015, currently Mar 2019) and we can drop support
for this version.

matterhorn-chat/matterhorn