.travis.yml: eliminate concurring build options

Eliminate the matrix description which overides the jobs.
If both keywords are present, matrix takes precedence. This made
subsequent stages specified in the jobs section to be ignored.

Fixes #452

.travis.yml

@@ -18,17 +18,6 @@ env:
   - PLATFORM=posix TESTS=yes TLS=tinydtls
   - PLATFORM=posix TESTS=yes TLS=mbedtls
 
-matrix:
-  exclude:
-    - compiler: clang
-      env: PLATFORM=posix TESTS=yes TLS=no
-    - compiler: clang
-      env: PLATFORM=posix TESTS=yes TLS=tinydtls
-    - compiler: clang
-      env: PLATFORM=posix TESTS=yes TLS=gnutls SMALL_STACK=yes EPOLL=no
-    - compiler: clang
-      env: PLATFORM=posix TESTS=yes TLS=gnutls SMALL_STACK=no  EPOLL=no
-
 before_install:
   - docker build -t obgm/libcoap:travis-env .
 
@@ -47,21 +36,17 @@ stages:
 jobs:
   include:
     - stage: other platforms
-      compiler: gcc
-      env:
-        - PLATFORM=contiki TLS=no
+      env: PLATFORM=contiki TLS=no
+      before_script:
       script:
         - docker run -e CC -e PLATFORM -e TLS obgm/libcoap:travis-env /bin/sh -c "scripts/build.sh"
     - stage: other platforms
-      compiler: gcc
-      env:
-        - PLATFORM=lwip TLS=no
+      env: PLATFORM=lwip TLS=no
+      before_script:
       script:
         - docker run -e CC -e PLATFORM -e TLS obgm/libcoap:travis-env /bin/sh -c "scripts/build.sh"
     - stage: dist
-      compiler: gcc
-      env:
-        - PLATFORM=posix TESTS=yes TLS=no DOCS=yes
+      env: PLATFORM=posix TESTS=yes TLS=no DOCS=yes
       before_script:
       script:
         - docker run -e CC -e PLATFORM -e TESTS -e DOCS -e TLS obgm/libcoap:travis-env /bin/sh -c "scripts/dist.sh"