travis: update and cleanup settings

Remove coverity settings since that's in its own branch
Remove old CMake PPA (Trusty has a more recent version)
Remove ccache from APT install since we build it from source
Disable caching ccache DB between builds
Don't run jobs on cron that won't do Make builds

.travis.yml

@@ -2,25 +2,13 @@ language: cpp
 sudo: false
 
 addons:
-  coverity_scan:
-    project:
-      name: "diydrones/ardupilot"
-      description: "Build submitted via Travis CI"
-    notification_email: andrew-scan@tridgell.net
-    build_command_prepend: "make clean"
-    build_command:   "make"
-    branch_pattern: coverity_scan
-    
   apt:
     sources:
-      - sourceline: 'ppa:george-edison55/cmake-3.x'
       - ubuntu-toolchain-r-test
-#      - george-edison55-cmake-3.x
 #      - llvm-toolchain-precise-3.7
     packages:
       - build-essential
       - gawk
-      - ccache
       - genromfs
       - libc6-i386
       - libxml2-dev
@@ -30,13 +18,13 @@ addons:
       - zlib1g-dev
       - gcc-4.9
       - g++-4.9
-      - cmake
-      - cmake-data
+      - cmake3
+      - cmake3-data
 #      - clang-3.7
 #      - llvm-3.7
 
 cache:
-  ccache: true
+  ccache: false
   directories:
     - $HOME/opt
 
@@ -47,9 +35,6 @@ before_install:
 script:
   - Tools/scripts/build_ci.sh
 
-before_cache:
-  - ccache -z
-
 notifications:
   webhooks:
     urls:
@@ -75,7 +60,9 @@ matrix:
       env: CI_BUILD_TARGET="px4-v2 revo-bootloader"
     - compiler: "gcc"
       env: CI_BUILD_TARGET="px4-v4 revo-mini"
-    - compiler: "gcc"
+    - if: type != cron
+      compiler: "gcc"
       env: CI_BUILD_TARGET="sitltest-copter"
-    - compiler: "gcc"
+    - if: type != cron
+      compiler: "gcc"
       env: CI_BUILD_TARGET="sitltest-quadplane sitltest-plane sitltest-rover"