Check re2 gem can be required

Before if the re2 gem were upgraded, it might silently segfault if error were used. There is now a Homebrew fix https://github.com/Homebrew/homebrew-core/commit/a0c61ee83a23ac6532455cf0129193d4e134af4b that now properly causes the gem to fail if the ABI changes. This was handled in https://github.com/mudge/re2/issues/43#issuecomment-622945994, so we can now simply check require 're2' as we do with other gems.