Install doxygen using apt

Downloading Doxygen 1.8.15 from its website is very, very slow, and the
doxygen package in Ubuntu's repositories no longer installs latex (or
maybe I made a mistake with that before?).

.travis.yml

@@ -6,6 +6,7 @@ addons:
     sources:
       - ubuntu-toolchain-r-test # ppa:ubuntu-toolchain-r/test
     packages:
+      - doxygen
       - libbz2-dev
       - libssl-dev
       - libhttp-parser-dev
@@ -28,12 +29,6 @@ install:
   - pip install --user -r docs/requirements.txt
   # Add sphinx-build to PATH
   - export PATH="$HOME/.local/bin:$PATH"
-  # Don't install Doxygen from the repositories, it also gets the LaTeX packages
-  # so is a ~ 700 MB download.
-  - wget http://doxygen.nl/files/doxygen-1.8.15.linux.bin.tar.gz
-  - tar -xf doxygen-1.8.15.linux.bin.tar.gz
-  # Add Doxygen binary path to PATH
-  - export PATH="$PWD/doxygen-1.8.15/bin:$PATH"
   # Install cbindgen for generating C++ headers for Rust dependencies.
   - cbindgen --version || cargo install cbindgen --version 0.6.6