add sympy to auto-installable requirements
remove double call to compile_boolean in boolean_combine

compile_boolean() already is called in BinaryOp case.
Otherwise it messes up the visitor pattern.
boolean expression visitor and use it to refactor compile_boolean
undo 1bda0dd
forgot the EQ abstract visit method
add a test for the visitor
Merge branch 'master' into bmyerz-refactor-boolean-traversal
Merge branch 'master' into bmyerz-refactor-boolean-traversal

Conflicts:
	raco/expression/expression.py
Merge pull request #109 from uwescience/bmyerz-refactor-boolean-traversal

Refactor boolean traversal