Explicitly state linux build requirements to hopefully avoid future

stall problems on Travis-CI.

.travis.yml

@@ -10,42 +10,56 @@ notifications:
     template:
       - "%{repository}/%{branch} (%{commit} - %{author}): %{build_url}: %{message}"
 
+sudo: required
+
 env:
   matrix:
     - TRAVIS_EMPTY_JOB_WORKAROUND=true
 
-os: linux
-
 matrix:
   exclude:
     - env: TRAVIS_EMPTY_JOB_WORKAROUND=true
   include:
     - env: TOOLSET=clang-3.4
+      os: linux
+      dist: precise
     - env: TOOLSET=clang-3.5
+      os: linux
+      dist: precise
     - env: TOOLSET=clang-3.6
+      os: linux
+      dist: precise
     - env: TOOLSET=gcc-4.7
+      os: linux
+      dist: precise
     - env: TOOLSET=gcc-4.8
+      os: linux
+      dist: precise
     - env: TOOLSET=gcc-4.9
+      os: linux
+      dist: precise
     - env: TOOLSET=gcc-5.1
-    - os: osx
-      env: TOOLSET=xcode-6.1
-    - os: osx
-      env: TOOLSET=xcode-6.2
+      os: linux
+      dist: precise
+    - env: TOOLSET=xcode-6.1
+      os: osx
+    - env: TOOLSET=xcode-6.2
+      os: osx
       osx_image: beta-xcode6.2
-    - os: osx
-      env: TOOLSET=xcode-6.3
+    - env: TOOLSET=xcode-6.3
+      os: osx
       osx_image: beta-xcode6.3
-    - os: osx
-      env: TOOLSET=xcode-6.4
+    - env: TOOLSET=xcode-6.4
+      os: osx
       osx_image: xcode6.4
-    - os: osx
-      env: TOOLSET=xcode-7.0
+    - env: TOOLSET=xcode-7.0
+      os: osx
       osx_image: xcode7
-    - os: osx
-      env: TOOLSET=xcode-7.1
+    - env: TOOLSET=xcode-7.1
+      os: osx
       osx_image: xcode7.1
-    - os: osx
-      env: TOOLSET=xcode-7.2
+    - env: TOOLSET=xcode-7.2
+      os: osx
       osx_image: xcode7.2
 
 before_install: wget "https://raw.githubusercontent.com/boostorg/regression/develop/ci/src/script.py"