Fixes #420

Base patched Trackler gem off actual version tag.

- was: hand-rolling a version of Trackler based of master and then
  applying changes from the PR/commit/local change to that trackler gem.
- now: using the exact tagged version of Trackler as the basis for the
  hand-rolled gem.

This fixes the issue where a change make to Trackler's master is
incompatible with the current version of x-api.  In short, we were
building the Trackler gem wrong.

nth-prime: simplify example implementation

Secret-handshake: remove whitespace

difference-of-squares: remove whitespace

pascals-triangle: update to v1.0.0 tests

Add script to prune extraneous .keep files

Initial configuration specifies that "first-timers only" issues should
be exempt from cleanup.

palindrome-products: make class use instance methods [Fix #377]

bookstore: fix test suite equality checks

atbash-cipher: make class use instance method [Fix #366]

Fixes issue #377.

Fixes issue #366.

INSTALLATION.md: Fixed spelling errors in documentation

flatten-array: update to v1.1.0 of canonical tests

pig-latin: convert to use instance methods

prime-factors: convert to use instance methods

nth-prime: make use instance method [Fix #358]

https://github.com/exercism/x-common/blob/d7a3c09b6758a8a5d59a2e140e6684031316280a/exercises/flatten-array/canonical-data.json

+ bookstore: add missing import statement in tests

twelve-days: use instance methods [Fix #357]

Beer song instance methods

Fixes issue #357.

Refactoring from static methods to instance methods

sum-of-multiples: use instance method instead of static method [Fix #355]

difference-of-squares: adjust API to use instance method [Fix #253]

Fixes issue #355.

Sieve redundant array

Switched from Arrays.asList to a Collections.singletonList

Fixed issue #359