Adds support for HEAD method. Removes rest-client gem

Gem is removed because it does not support HTTP HEAD request method

Adds support for HEAD method via Ruby stdlib class Net::HTTP

Changes valid response code to be in range of 100-399, not 200 only

Adds option to setup ping request method and interval (in seconds)
  via environmental variables, e.g.

  PING_METHOD = 'HEAD' (default) or 'GET'
  PING_INTERVAL = '1200' (default)

Signed-off-by: Aleksandrs Ļedovskis <aleksandrs@ledovskis.lv>