travis: Extended kernel build matrix

When doing test builds, build against the following kernels:
  - KERNEL=3.17.4
  - KERNEL=3.16.7
  - KERNEL=3.14.25
  - KERNEL=3.12.33
  - KERNEL=3.10.61
  - KERNEL=3.4.104
  - KERNEL=2.6.32.64

When doing pure kernel test builds, skip compilation of user space bits.

Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>