Use tilde in version spec

Was getting an error when trying to install package, with composer
returning that it could not find a version of "wp-cli" that matched
"v0.9.4". Upon checking packagist, found that this version was patched
and now called "v0.9.4-patch46". Adding the tilde in front of the
current version will allow this to download properly and if patched
again, remain working seamlessly since it sets "v0.9.4" to a minimum
allowable version.