Update dependancies

[josephlr]

This make the following updates to our dependancies:

• go-tpm: 0.3.2 -> 0.3.3
  • PolicySecret now returns three values (Add TPM2_PolicySigned and Signature encoding go-tpm#237)
  • We never read the encoded value from Certify, so we are not affected by Various fixes for Remote Attestation implementations go-tpm#235
• go-attestation: 0.4.0 -> 0.4.2
  • We also no longer build with TPM 1.2 support by default, so we don't need to install libtspi anymore!!
• go-cmp: 0.5.6 -> 0.5.7
• cobra: 1.1.3 -> 1.3.0