Docker build no longer works with github url

Change default URL for progrium/buildstep to include full git clone url
Merge pull request #405 from AlJohri/master

Fix docker build for docker v0.7.3
Add $DOCKER_VERSION to specify docker version
Suppress git advice related to detached HEAD
Merge pull request #400 from aegypius/suppress-git-output-unless-dokku-trace-is-enabled

Suppress git "detached HEAD" notice output
Merge pull request #407 from banyan/specify-docker-version

Add $DOCKER_VERSION to specify docker version
Fix typo in README.md

aditional -> additional
Merge pull request #415 from JasonPunyon/patch-1

Fix typo in README.md
Bump version number in dokku-installer.rb

Just bumping the version number to bring it up-to-date with the rest of the project.
Recommend 12.04 due to 13.04's EOL
SSL -> TLS
update copyright year
Added option to set memory size for vagrant
Merge pull request #454 from dmyers/vagrant-memory

Added option to set memory size for vagrant
Merge pull request #443 from gdi2290/patch-1

update copyright year
Merge pull request #440 from imchairmanm/patch-1

Bump version number in dokku-installer.rb
Using logs --follow instead of attaching to instance
If passing -t for tail, use logs --follow else just get last 100 lines
Merge pull request #467 from silasb/fixing-log-tail

Using logs --follow instead of attaching to instance
Update stable's version