1. A download-build-imagemagick.sh script that automates downloading and
   building static libraries for ImageMagick and dependencies from source on
   MacOS and Linux/CentOS6.

2. A Vagrantfile that defines 3 boxes for building ImageMagick static libraries
   from source, testing that the go tests work on linux, and verifying the
   resulting binary runs in production.

3. The precompiled libraries resulting from the above steps, so that clients on
   either platform can use them without effort.

4. Tweaks to magick_wand_test.go to match the current usage. Specific image
   formats are verified to be present, and tests for font support are disabled.

5. #cgo directives in magick_wand.go so that the precompiled libraries are used
   by the Go command line tool as well. Also add a -Wno-ignored-attributes to
   suppress a noisy warning from include/magick/memory_.h:31

Support Go 1.13

In order for the generated libraries to be linkable in Ubuntu and
runnable in CentOS, they must be compiled as Position-Independent
Executables (PIE). This is done by injecting the -fPIE cflag in when
compiling ImageMagick. The updated object files in this commit were
generated with this flag added.

Additional changes were made to documentation and Vagrantfile
in order to test building in Ubuntu, linking in Ubuntu via Bazel, and
running the final binary in CentOS.

The version number referenced in the README was also corrected to match
the version downloaded/built in download-build-imagemagick.sh.

Added Ubuntu support by recompiling with -fPIE

This involved the following changes:

1. Upgrade libwebp to 1.1
2. Add libltdl
3. Patch wand/pixel-wand.h to include the PixelWand struct definition.
   It's instantiated in Go, and it's not possible to allocate an
   incomplete struct.

Make some other small improvements:

- Update rules_go to latest.
- Add a .bazelrc with some helpful settings.
- Add some helpful dev tips to README.
- Patch download-build-imagemagick

Tested by following the instructions in README.