Add installation steps for new version of cmake

The Travis-preinstalled CMake is unfortunately not installed via apt, so we
can't replace it with an apt-supplied version very easily. Additionally, we
can't permit the Travis-preinstalled copy to survive, as the Travis default
path lists the Travis CMake install location ahead of any place where apt
would install CMake to. Instead of apt removing or upgrading to a new CMake
version, we must instead delete the Travis copy of CMake and install a
newer version via apt.

Author: harrisonmutai-arm

.travis.yml

@@ -35,8 +35,13 @@ before_install:
 
 addons:
   apt:
+    sources:
+      - sourceline: 'deb https://apt.kitware.com/ubuntu/ xenial main'
+        key_url: 'https://apt.kitware.com/keys/kitware-archive-latest.asc'
+      - sourceline: 'deb https://apt.kitware.com/ubuntu/ xenial-rc main'
     packages:
-    - ninja-build
+      - cmake
+      - ninja-build
 
 matrix:
   include:
@@ -270,6 +275,14 @@ matrix:
       language: python
       python: 3.7
       install:
+        # Hide Travis-preinstalled CMake
+        # The Travis-preinstalled CMake is unfortunately not installed via apt, so we
+        # can't replace it with an apt-supplied version very easily. Additionally, we
+        # can't permit the Travis-preinstalled copy to survive, as the Travis default
+        # path lists the Travis CMake install location ahead of any place where apt
+        # would install CMake to. Instead of apt removing or upgrading to a new CMake
+        # version, we must instead delete the Travis copy of CMake.
+        - sudo rm -rf /usr/local/cmake*
         # Install gcc
         - source_pkg gcc
         - arm-none-eabi-gcc --version
@@ -347,8 +360,10 @@ matrix:
       name: "cmakecheck"
       env: NAME=cmakecheck
       language: python
-      python: 3.7
+      python: 3.8
       install:
+        # Hide Travis-preinstalled CMake
+        - sudo rm -rf /usr/local/cmake*
         # Install python modules
         - python -m pip install --upgrade pip==18.1
         - python -m pip install --upgrade setuptools==40.4.3