- Add a usage/network.rst document for all network related information (fixes #1852)
- reference it from cs2cs and cct docs
- Create a specification/ section
- Move usage/projjson.json to specification/
- Move the description of the GeoTIFF format from RFC-4 text to a
  specifications/geodetictiffgrids.rst (refs #1850)

* From openjdk-7 via apt to openjdk-11 via Travis CI xenial's $JAVA_HOME
* From mingw 4.8 -> 5.3, as supported by xenial
* clang+llvm for csa kept at version 6.0.0, but for ubuntu-16.04
* Furthermore xenial implicitly has many upgrades, specifically:
  - gcc 4.8.4 -> 5.4.0
  - clang 4.0.0 -> 5.0.0
  - cmake 3.9.2 -> 3.12.4
  - python 3.4.3 -> 3.5.2
* cppcheck version kept at 1.61 (for trusty) due to many
  false positives with version 1.72 (for xenial)
* Community code contribution documentation simplified to remove
  version of cppcheck, as this may outdated with updates to scripts

* Split global before_install for apt and pip
* Use -qq option for apt-get, which implies -y
* Use list of targets with apt-get rather than one-per-line
* Use local variables for mingw32/install.sh
* Show version info for cppcheck, pip and python
* Simplify clang+llvm-6 path

More or less all old questions from the FAQ has been removed and
replaced by new questions that align better with the current state of
the project.

RFC3: Dependency management