Make libffi vendoring check for existing directory in correct location

For both the cedar and cedar-14 stacks, the libffi directory is named
`libffi-3.1` not `libffi-3.1.1`, so the existing library wouldn't be
detected, causing it to be re-downloaded during every compile.

bin/steps/cryptography

@@ -22,7 +22,7 @@ bpwatch start libffi_install
 # If a package using cffi exists within requirements, use vendored libffi.
 if (pip-grep -s requirements.txt bcrypt cffi cryptography django[bcrypt] Django[bcrypt] PyNaCl pyOpenSSL PyOpenSSL requests[security] misaka &> /dev/null) then
 
-  if [ ! -d ".heroku/vendor/lib/libffi-3.1.1" ]; then
+  if [ ! -d ".heroku/vendor/lib/libffi-3.1" ]; then
     echo "-----> Noticed cffi. Bootstrapping libffi."
     mkdir -p .heroku/vendor
     # Download and extract libffi into target vendor directory.