Fix incorrect watermark CRITICAL due to new JSON version

The newly released version 2.90 of JSON.pm on CPAN means that
check_rabbitmq_watermark will go CRITICAL on both "mem_alarm" and "disk_alarm".

This is due to the fact that a submodule used by JSON.pm has broken
compatibility with the feature that allowed comparisons of JSON booleans to
'true' and 'false' in Perl:

    http://search.cpan.org/~makamaka/JSON/lib/JSON.pm#DESCRIPTION

Copy & paste of the warning from JSON.pm's release notes:

>
>   INCOMPATIBLE CHANGE (JSON::XS version 2.90)
>
>   JSON.pm had patched JSON::XS::Boolean and JSON::PP::Boolean internally
>   on loading time for making these modules inherit JSON::Boolean.
>   But since JSON::XS v3.0 it use Types::Serialiser as boolean class.
>   Then now JSON.pm breaks boolean classe overload features and
>   -support_by_pp if JSON::XS v3.0 or later is installed.
>
>   JSON::true and JSON::false returned JSON::Boolean objects.
>   For workaround, they return JSON::PP::Boolean objects in this version.
>
>       isa_ok(JSON::true, 'JSON::PP::Boolean');
>
>   And it discards a feature:
>
>       ok(JSON::true eq 'true');
>
>   In other word, JSON::PP::Boolean overload numeric only.
>
>       ok( JSON::true == 1 );
Merge pull request #25 from martinmelin/fix-watermark-check-broken-by-json-update

Fix incorrect watermark CRITICAL due to new JSON version.

thanks for the patch!