Disallow CMake build failures on Travis

Our CMake configuration has matured over time, thus require CMake
builds to require successful builds on Travis CI.

.travis.yml

@@ -16,11 +16,6 @@ env:
     - secure: "OI9CUjj4lTb0HwwIZU5PbECU3hLlAL6KC8KsbwohG8/O3j5fLcnmDsK4Ad9us5cC39sS11Jcd1kDP2qRcCuST/glVNhLkcjKkiQerOfd5nQ/qL4JYfz/1mfP5mdpz9jHKzpLUIG+TXkbSTjP6VVmsb5KPT+3pKEdRFZB+Pu9+J8="
     - coverity_branch: coverity_scan
 
-matrix:
-  allow_failures:
-    - env: libt_branch=RC_1_2 gui=true build_system=cmake
-    - env: libt_branch=RC_1_2 gui=false build_system=cmake
-
 branches:
   except:
     - search_encoding_windows