core: use Ubuntu Trusty for tests (Travis CI)

The Debian package libcpputest-dev becomes useless as it's available in
the repositories now.

.travis.yml

@@ -4,6 +4,9 @@ compiler:
     - gcc
     - clang
 
+sudo: required
+dist: trusty
+
 env:
     - BUILDTOOL="cmake" BUILDARGS=""
     - BUILDTOOL="cmake" BUILDARGS="-DENABLE_PYTHON3=ON"
@@ -13,10 +16,11 @@ env:
     - BUILDTOOL="autotools" BUILDARGS="--disable-gnutls"
 
 before_script:
+    # Workaround travis-ci/travis-ci#5326
+    - export PATH=$(echo $PATH | tr ':' "\n" | sed '/\/opt\/python/d' | tr "\n" ":" | sed "s|::|:|g")
     - echo 'APT::Install-Recommends "false";' | sudo tee -a /etc/apt/apt.conf
     - sudo apt-get update -qq
-    - sudo apt-get -y install devscripts equivs python-pip libenchant-dev autopoint asciidoc source-highlight xsltproc docbook-xsl docbook-xml cmake pkg-config libncursesw5-dev gem2deb libperl-dev python-dev python3-dev libaspell-dev liblua5.1-0-dev tcl8.5-dev guile-2.0-dev libv8-dev libcurl4-gnutls-dev libgcrypt11-dev libgnutls-dev zlib1g-dev curl
-    - sudo dpkg -i ./tests/ubuntu/precise/libcpputest-dev_3.4-3_amd64.deb
+    - sudo apt-get -y install devscripts equivs python-pip libenchant-dev autopoint asciidoc source-highlight xsltproc docbook-xsl docbook-xml cmake pkg-config libncursesw5-dev gem2deb libperl-dev python-dev python3-dev libaspell-dev liblua5.1-0-dev tcl8.5-dev guile-2.0-dev libv8-dev libcurl4-gnutls-dev libgcrypt11-dev libgnutls-dev zlib1g-dev curl libcpputest-dev
     - sudo pip install msgcheck pylint
 
 script: