Bump version numbers to v0.3.0

[Kijewski]

• rinja 0.2.1 is backward compatible to v0.2.0, only adding new functions traits, not changing existing ones
• rinja_derive 0.3.0 emits code that uses e.g. rinja::helpers::get_primitive_value() which were not present in rinja 0.2.0, so it is not backward compatible
• rinja_parser introduces new enum members, so it is not backward compatible

Resolves #101.

[GuillaumeGomez]

I'd prefer if we kept the same version number across all crates. Makes it simpler for users to know what's going on instead of trying to remember which version of X maps to Y.

[Kijewski]

Then we must either

• bump the version of rinja to 0.3.0, too, even though it is not a breaking change, or
• pin the rinja_derive version in rinja, and the rinja_parser version in rinja_derive, but accept that people who update only one of the dependencies are in for trouble, or
• do both, so for later releases we can update the pinned version, making later releases easier.

Then I'd prefer the 3rd solution:

rinja/Cargo.toml: version = "0.3.0", rinra_derive = "=0.3.0"
rinja_derive/Cargo.toml: version = "0.3.0", rinra_parser = "=0.3.0"
rinja_parser/Cargo.toml: version = "0.3.0"

[GuillaumeGomez]

I'd update all versions to 0.3.0 so all rinja crates are equal (in version) to each others.

[GuillaumeGomez]

Same here and below.