test/: rename everything in accordance with Pry project conventions

Well, the conventions are implicit.
Rakefile: include "CHANGELOG.md" into the list of files
Rakefile: update the "test" task to new path
PryDocSpec: update "require" call to new path
Rakefile: add alias "spec" for the "test" task
.gitignore: ignore "Gemfile.lock"
Gemfile: update; avoid Bundler warning
PryDocSpec: add tests for specific Ruby versions

Just basic tests to ensure that generated docs aren't messed up. Every
single test tests a specific feature of the currently running Ruby.
Regenerate Core docs 1.8

Based on ruby-1.8.7-p370.
CHANGELOG.md: add information about 1.8.7 docs