Problem: Travis installs packages when not needed

Solution: narrow down the matrix to install valgrind and lcov only
for the test runs that use them.
CMake is part of the default image so no need to install it at all.

.travis.yml

@@ -14,30 +14,28 @@ env:
 - BUILD_TYPE=cmake CURVE=tweetnacl
 - BUILD_TYPE=default CURVE=libsodium
 - BUILD_TYPE=default
-- BUILD_TYPE=coverage CURVE=tweetnacl DRAFT=enabled
-- BUILD_TYPE=valgrind CURVE=tweetnacl DRAFT=enabled
 
 matrix:
-  exclude:
-  - env: BUILD_TYPE=coverage CURVE=tweetnacl DRAFT=enabled
-    os: osx
-  - env: BUILD_TYPE=valgrind CURVE=tweetnacl DRAFT=enabled
-    os: osx
   include:
   - env: BUILD_TYPE=default CURVE=tweetnacl IPv6=ON
     os: linux
     dist: precise
     sudo: false
-
-sudo: required
-
+  - env: BUILD_TYPE=coverage CURVE=tweetnacl DRAFT=enabled
+    os: linux
     addons:
       apt:
         packages:
-    - cmake
         - lcov
+  - env: BUILD_TYPE=valgrind CURVE=tweetnacl DRAFT=enabled
+    os: linux
+    addons:
+      apt:
+        packages:
         - valgrind
 
+sudo: required
+
 before_install:
 - if [ $TRAVIS_OS_NAME == "osx" ] ; then brew update; brew install binutils ; fi
 # workaround for Travis OSX CI bug, hasn't been fixed in a month so time for a hack