[5.5] Allow single error messages in ValidationException::withMessages

[sebastiandedeyne]

Allows this:

throw ValidationException::withMessages([
    'email' => 'This address is never allowed to be registered',
]);

Instead of always having to do this:

throw ValidationException::withMessages([
    'email' => ['This address is never allowed to be registered'],
]);

[themsaid]

@sebastiandedeyne whoops, Travis failing.

[sebastiandedeyne]

Hmm doesn't seem to be related with this change:

$ phpenv config-rm xdebug.ini || true
File xdebug.ini removed from /home/travis/.phpenv/versions/7.1/etc/conf.d.
$ echo "extension = memcached.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
$ printf "\n" | pecl install -f redis
Package "redis" does not have REST info xml available
install failed

Just merged so guess it's okay 😄