v0.1.5-Bugfix: png generation fails in a Lambda environment

In an AWS Lambda container, no paths are writable except /tmp. When
generating PNGs via inframap/dot, the program attempted to write to
$XDG_CACHE_HOME, which was /validiac/bin/.cache, i.e. a non-writable
path. This commit fixes the issue by setting $XDG_CACHE_HOME to
/tmp/.cache instead.

In addition, core MS fonts are installed in the image so that Graphviz
can properly render text.

Also, the Lambda handlers will now encode output in base64 encoding when
the output is binary rather than textual. It should be noted that API
Gateway will only decode base64 responses for media types that were
defined in the API configuration's "Binary Media Type" list, and when
clients send appropriate Accept headers.