Bump phf_generator generator version to 0.7.22
Bump phf_generator generator version to 0.7.22

This is useful for cargo minimal version builds. `phf_generator`
in versions before 0.7.22 depended on rand 0.3 which itself
depended on libc 0.1.1 that no longer compiles with the newest
Rust version.
servo/string-cache