switch truncate_digest_for_ecdsa to context manager

Also adds a docstring to fix #1335
Merge pull request #1349 from reaperhulk/more-ctx-mger

switch truncate_digest_for_ecdsa to context manager
Added missing vector file
fixes #1340 -- use pypy 2.4.0 now that it's out
Merge pull request #1350 from michael-hart/ec_vectors

Added missing vector file
bump pyenv version
bleh update first
Merge pull request #1351 from alex/pypy-2.4

fixes #1340 -- use pypy 2.4.0 now that it's out
deprecate backend method names for elliptic curve number loading

fixes #1270
skip deprecated tests on platforms that don't support ec (old rhel)
multibackend deprecated method needs to call the backend deprecated method
fix test coverage on multibackend deprecated methods
return from deprecated method
remove duplicate _skip_curve_unsupported