Disable trusty builds on Travis

Ubuntu Trusty doesn't ship with a new enough pkg-config, which causes
the PKG_INSTALLDIR macro to fail on those systems. Since Trusty is EOL
anyway, simply disable the CI build on it.

.travis.yml

@@ -13,12 +13,6 @@ compiler:
 
 jobs:
     include:
-        - os: linux
-          dist: trusty
-          compiler: gcc
-        - os: linux
-          dist: trusty
-          compiler: clang
         - os: linux
           dist: xenial
           compiler: gcc

README.md

@@ -20,7 +20,7 @@ the individual folders for README information.
 
 Build has been tested on the following operating systems (x86-64 only):
 
-* Ubuntu 14.04 LTS
+* ~~Ubuntu 14.04 LTS~~
 * Ubuntu 16.04 LTS
 * Ubuntu 18.04 LTS
 * OS X 10.13.6