Update opensaml dependency to latest version

[dmogn]

Related with #26.
Maybe latest opensaml version has more parsing capabilities for unusual attributes, like google IDP.

[nynymike]

What makes these attributes unusual?

[mzico]

@nynymike : https://support.gluu.org/identity-management/google-idp-custom-attributes-seem-to-trigger-500-server-error-from-asimba-3260/

[dmogn]

@nynymike opensaml cannot parse/extract attr value in this case.
We should switch to latest opensaml version before next steps, I think.

[dmogn]

opensaml does not show errors, but parse empty attr value

[dmogn]

Current dependency version: 2.6.1
Latest V2: 2.6.4
V2 branch support discontinued at july 2016.
The current stable release is 3.2.0

[dmogn]

It will be big change, because of API difference. But opensaml v2 is obsolete and unsupported, so we need do it.

[nynymike]

Note from Mark Dobrinic:

Only thing I remember is when going from OpenSAML 2.5.x to 2.6.x, 
there was this thing with the MetadataProviders, taking a thread per 
provider. That was quite some thing to integrate.`