Remove "rubythemis" gem (#508)

"rubythemis" name has been deprecated in favor of "rbthemis" since
Themis 0.11 and is scheduled for removal in 0.12 which is close.
Drop the old gem, deprecation warnings, and Makefile targets.

Makefile

@@ -472,13 +472,6 @@ ifdef RUBY_GEM_VERSION
 	@$(BUILD_CMD_)
 endif
 
-rubythemis_uninstall_deprecation_warning:
-	@printf "The rubythemis gem is deprecated. Please use rbthemis_uninstall$(WARN_STRING)\n"
-	@printf "target instead of rubythemis_uninstall.$(WARN_STRING)\n"
-
-rubythemis_uninstall: rubythemis_uninstall_deprecation_warning rbthemis_uninstall
-
-
 jsthemis_uninstall: CMD = rm -rf build/jsthemis-$(JSTHEMIS_PACKAGE_VERSION).tgz && npm uninstall jsthemis
 jsthemis_uninstall:
 ifdef NPM_VERSION
@@ -518,12 +511,6 @@ else
 	@exit 1
 endif
 
-rubythemis_install_deprecation_warning:
-	@printf "The rubythemis gem is deprecated. Please use rbthemis_install$(WARN_STRING)\n"
-	@printf "target instead of rubythemis_install.$(WARN_STRING)\n"
-
-rubythemis_install: rubythemis_install_deprecation_warning rbthemis_install
-
 jsthemis_install: CMD = cd src/wrappers/themis/jsthemis && npm pack && mv jsthemis-$(JSTHEMIS_PACKAGE_VERSION).tgz ../../../../build && cd - && npm install nan && npm install ./build/jsthemis-$(JSTHEMIS_PACKAGE_VERSION).tgz
 jsthemis_install:
 ifdef NPM_VERSION