all: Update voms-api to 2.0.6

We are using an unsupported and buggy voms-api version
Switch to voms-api to 2.0.6 fixing "Certificate verification: Maximum certification path length exceeded." exception problem

Ticket: http://rt.dcache.org/Ticket/Display.html?id=7848
Acked-by: Gerd
Target: 2.2
Require-book: no
Require-notes: yes
Merge pull request #124 from chrber/fix/2.2/rb5712

all: Update voms-api to 2.0.6