bin/build: Remove unneeded files after build

This is intended to replace the functionality of the .slugignore file,
by deleting files which aren't strictly required to run the
server *after* building. The reason for this is because any files in the
slugiginore file are deleted *prior* to invoking bin/build, which in our
case breaks autogen:

    $ ./autogen.sh
    Checking whether you have the necessary tools...
    Checking for autoconf (need at least version 2.65)... ok
    Checking for automake (need at least version 1.11)... ok
    Your system has the required tools, running autoreconf...
    configure.ac:422: required file `Documentation/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/pages/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/pages/renders/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/previous/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/previous/renders/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/tasks/Makefile.in' not found
    configure.ac:422: required file `Documentation/design/d1/tasks/renders/Makefile.in' not found
    configure.ac:422: required file `Documentation/plan/Makefile.in' not found
    configure.ac:435: required file `tools/Makefile.in' not found
    configure.ac:435: required file `tools/dsa.sh.in' not found
    configure.ac:441: required file `scripts/Makefile.in' not found
    configure.ac:441: required file `scripts/git/Makefile.in' not found
    configure.ac:441: required file `scripts/git/hooks/Makefile.in' not found
    configure.ac:441: required file `scripts/git/hooks/pre-commit.in' not found
    Makefile.am:1: required directory ./scripts does not exist
    Makefile.am:7: required directory ./Documentation does not exist
    Makefile.am:11: required directory ./tools does not exist
    autoreconf: automake failed with exit status: 1

Issue #326.
Revert "Add a Heroku .slugignore file"

This reverts commit b67b85c48fefd4c1c4b9f678beca9e9e3e2f5b0f.

Issue #326.
bin/build: Make output text a little clearer

Issue #326.
Bump release version for '0.5.5'
Merge branch 'release/0.5.5'