Ensure that CHECK constraints are surrounded with parentheses

Previously, Sequel didn't enforce the use of parentheses, but most
filter conditions in Sequel use parentheses, so for most common
cases it didn't matter.

This ensures the check constraints always have surrounding
parentheses.  Since I'm not sure how all databases handle multiple
levels of nested parentheses in check constraints, I've decided to be
conservative and only add parentheses if the expression doesn't
already appear to be surrounded by them.
Fix spec nondeterminism, caught by Travis
Format BigDecimal values like Floats in pretty_table extension

The previous formatting was not user friendly.