Fix Travis build environment.

Newest Meson requires Python 3.5.which isn't available in Trusty.
Pip version pin no longer necessary.

Author: Nikratio

.travis.yml

@@ -14,8 +14,6 @@ addons:
     - gcc-6
     - python-docutils
     - python3-pip
-before_install:
-   - sudo -H python3 -m pip install pip==8.1.1
 install: test/travis-install.sh
 script: test/travis-build.sh
 

test/travis-install.sh

@@ -2,7 +2,8 @@
 
 set -e
 
-sudo python3 -m pip install pytest meson
+# Meson 0.45 requires Python 3.5 or newer
+sudo python3 -m pip install pytest meson==0.44
 wget https://github.com/ninja-build/ninja/releases/download/v1.7.2/ninja-linux.zip
 unzip ninja-linux.zip
 chmod 755 ninja