refactor(DssVestTransferrable): fix Certora spec

Makefile

@@ -8,7 +8,7 @@ echidna-suckable      :; ./echidna/echidna.sh suckable
 echidna-transferrable :; ./echidna/echidna.sh transferrable
 certora-mintable      :; PATH=${PATH} certoraRun certora/DssVestMintable.conf $(if $(rule), --rule $(rule),)
 certora-suckable      :; PATH=${PATH} certoraRun certora/DssVestSuckable.conf $(if $(rule), --rule $(rule),)
-certora-transferrable :; $(if $(CERTORAKEY),, @echo "set certora key"; exit 1;) certoraRun --solc ~/.solc-select/artifacts/solc-0.6.12/solc-0.6.12 src/DssVest.sol:DssVestTransferrable certora/Dai.sol --link DssVestTransferrable:gem=Dai --verify DssVestTransferrable:certora/DssVestTransferrable.spec --solc_args "['--optimize','--optimize-runs','200']" --rule_sanity  $(if $(rule),--rule $(rule),) --multi_assert_check --short_output
+certora-transferrable      :; PATH=${PATH} certoraRun certora/DssVestTransferrable.conf $(if $(rule), --rule $(rule),)
 deploy-mintable       :; make && dapp create DssVestMintable $(gem)
 deploy-suckable       :; make && dapp create DssVestSuckable 0xdA0Ab1e0017DEbCd72Be8599041a2aa3bA7e740F
 deploy-transferrable  :; make && dapp create DssVestTransferrable $(owner) $(gem)

certora/DssVestTransferrable.conf

@@ -0,0 +1,32 @@
+{
+  "files": [
+    "src/DssVest.sol:DssVestTransferrable",
+    "certora/mocks/Dai.sol",
+  ],
+  "link": [
+    "DssVestTransferrable:gem=Dai"
+  ],
+  "rule_sanity": "basic",
+  "multi_assert_check": true,
+  "wait_for_results": "all",
+  "solc_map": {
+    "DssVestTransferrable": "solc-0.6.12",
+    "Dai": "solc-0.6.12"
+  },
+  "solc_optimize_map": {
+    "DssVestTransferrable": "200",
+    "Dai": "0"
+  },
+  "verify": "DssVestTransferrable:certora/DssVestTransferrable.spec",
+  "prover_args": [
+    "-smt_hashingScheme plainInjectivity",
+    "-s [yices,z3]",
+    "-splitParallel true",
+    "-mediumTimeout 3600",
+    "-depth 30",
+    "-enableSolidityBasedInlining true"
+  ],
+  "parametric_contracts": [
+    "DssVestTransferrable"
+  ]
+}