fix to make specs run under 1.9.3

[ChrisLundquist]

needed to run specs in 1.9.3.

Otherwise you get an error something like

can't convert Pathname into String (TypeError)

ENV['CONFIG_PATH'] = @root.join('features/support/config/default').to_s

• Added rrd-ffi to gem dependencies. as discussed https://github.com/auxesis/visage/issues/76

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NOTE: you still need to install librrd-dev{el} via package manager. otherwise you get an error like

Could not open library 'rrd': rrd: cannot open shared object file: No such file or directory.
Could not open library 'librrd.so': librrd.so: cannot open shared object file: No such file or directory (LoadError)

Alternatively you can use the rrdlib gem and get a compile error when building native extensions ( when the library is missing).

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NOTE: your tests depend on collectd

No such file or directory - /usr/share/collectd/types.db (Errno::ENOENT) 

[ChrisLundquist]

@auxesis thank you!