quoted-strings: Fix explicit octal recognition

[adrienverge]

PyYAML implements YAML spec version 1.1, not 1.2. Hence, values starting
with 0o are not considered as numbers: they are just strings, so they
need quotes when quoted-strings: {required: true}.

>>> import yaml
>>> yaml.resolver.Resolver().resolve(yaml.nodes.ScalarNode, '100', (True, False))
'tag:yaml.org,2002:int'
>>> yaml.resolver.Resolver().resolve(yaml.nodes.ScalarNode, '0100', (True, False))
'tag:yaml.org,2002:int'
>>> yaml.resolver.Resolver().resolve(yaml.nodes.ScalarNode, '0o100', (True, False))
'tag:yaml.org,2002:str'

Let's try to prevent that.

Fixes #351.

[coveralls]

Coverage increased (+0.002%) to 97.922% when pulling 1fa0c4a on fix/quoted-strings-explicit-octal-values into ee4d163 on master.