The GPG signing subkey and passphrase are respectively provided through
a Travis encrypted file and a Travis encrypted environment variable.
Signing is only done if there is a signing key present when the build is
complete (so as to not fail when e.g. building a PR that doesn't have
encrypted files available).
Travis uses Ubuntu Precise, which has CMake 3.8. That version does not
have support for excluding /usr and /usr/bin from the %files% list
(which results in a package that conflicts with the filesystem package
and fails to install).
This commit:
+ Builds on Precise instead of Trusty
+ Adds install tests