removed `pyvows.assertions`
Replaced references to `expect` `decorators._assertion`, and `decorators._create_assertions` to preggy counterparts
tests.*: Updated test string checking to match updated preggy

The commit of “updated preggy” is:
 f7a9e617b9b842ee9f07641bd2f506c40fcc4469
reporting.common: better checking to format tracebacks
Updated requirements in project metadata

Updated in:

- Makefile
- REQUIREMENTS
- setup.py
- test_requirements.txt
- tox.ini
reporting.test: commented out problematic `except:` block
__init__: Removed import for `VowsAssertionError`
Updated for preggy’s module-level decorators
Removed `utils.VowsAssertion` class

Functionality now covered by preggy.
Moved `Vows.NotErrorContext` and `Vows.NotEmptyContext` to `tests/assertions/assertion_vows.py`

The only place they were ever used…
tests.assertions.assertion_vows: Minor rearrangement of sub contexts
Removed `_AssertionNotFoundError` class

Functionality now covered by preggy.
decorators: renamed variables in `_batch`

Silly rabbit!  `@Vows.batch` is for `class`es!
Fixed broken test `file_vows.py`
setup.py: Updated preggy version requirement
Merge pull request #93 from Zearin/refactor/#92-preggy

Close #92: Refactored PyVows to use preggy (an assertion library extracted from PyVows)
setup.py: minor tweaks