Modern Rust build fixes + Travis build

[palfrey]

So starting from the issues in #18, I fixed all the build issues I could find (both errors and warnings) and then got a Travis build up and running for both Ubuntu and OS X.

Notes:

• https://travis-ci.org/mheese/rust-pkcs11 will need activating and the build badge urls swapped over. https://travis-ci.org/palfrey/rust-pkcs11 shows builds so far.
• I wasn't able to get the build working with libsofthsm2 2.2 which is the default in Ubuntu Bionic, so there's an additional apt line in the build to pull in libsofthsm 2.4 from Disco. I don't quite know whether this is not meant to work, or if there's something I was missing in the testing.
• In order to make the tests work, the Travis user needs to be (in Linux at least) in the softhsm group, which then needs the sudo/su hack to get the group membership properly picked up.

[mheese]

@palfrey awesome!! thanks! ... I will look into this on the weekend in my spare time

[palfrey]

@mheese How's this getting on? Anything I can do to help?

[mheese]

@palfrey dealing with all the PRs right now ... this one is critical tbh ... thanks again

I believe that libsofthsm2 2.2 is actually implementing an older PKCS11 version and that might not work 🤔

As an additional feature we should probably test several different PKCS11 versions.

[mheese]

lgtm! 👍