Updated .travis.yml: switched to bionic build env

.travis.yml

@@ -1,13 +1,15 @@
 sudo: required
-dist: trusty
+dist: bionic
+
+addons:
+  apt:
+    packages:
+    - g++
+    - cmake
+    - libeigen3-dev
+    - libglew-dev
 
 before_install:
-  - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
-  - sudo apt -qq update && sudo apt install g++-7 libglew-dev
-  - sudo unlink /usr/bin/gcc && sudo ln -s /usr/bin/gcc-7 /usr/bin/gcc
-  - sudo unlink /usr/bin/g++ && sudo ln -s /usr/bin/g++-7 /usr/bin/g++
-  - wget https://gitlab.com/libeigen/eigen/-/archive/master/eigen-master.zip -O eigen.zip && unzip eigen.zip
-  - mkdir -p eigen-master/build && cd eigen-master/build && cmake .. && make -j8 && sudo make install && cd ../..
   - wget https://github.com/stevenlovegrove/Pangolin/archive/master.zip -O Pangolin.zip && unzip Pangolin.zip
   - mkdir -p Pangolin-master/build && cd Pangolin-master/build && cmake .. && make -j8 && sudo make install && cd ../..