The match obtained by applying the regular expression used to index into
the `stderr` member of the result of calling `system_command` in the relevant
variable assignment could yield unexpected results. The regular expression in-
volved was not strict enough and contained an unescaped period, which could
match any character, not just the expected literal decimal point. This commit
corrects this oversight by escaping the relevant character, thus addressing
@apjanke's remark in https://github.com/Homebrew/brew/pull/5280#issuecomment-437165119 on the existence of a:
> …possible bug - that `.` looks like it should be escaped as `\.` to match a
> literal ".".) …
33c3faa125