Pin version of pg to most recent 6.X.X version

Sequelize currently does not work with version 7 of pg. See
sequelize/sequelize#7998.

Fixes #1.

package.json

@@ -11,7 +11,7 @@
     "format": "prettier --write *.js"
   },
   "dependencies": {
-    "pg": "^7.0.2",
+    "pg": "^6.1.0",
     "sequelize": "^4.4.2",
     "umzug": "^2.0.1"
   },

yarn.lock

@@ -209,6 +209,10 @@ figures@^1.7.0:
     escape-string-regexp "^1.0.5"
     object-assign "^4.1.0"
 
+generic-pool@2.4.3:
+  version "2.4.3"
+  resolved "https://registry.yarnpkg.com/generic-pool/-/generic-pool-2.4.3.tgz#780c36f69dfad05a5a045dd37be7adca11a4f6ff"
+
 generic-pool@^3.1.6:
   version "3.1.7"
   resolved "https://registry.yarnpkg.com/generic-pool/-/generic-pool-3.1.7.tgz#dac22b2c7a7a04e41732f7d8d2d25a303c88f662"
@@ -291,6 +295,10 @@ isexe@^2.0.0:
   version "2.0.0"
   resolved "https://registry.yarnpkg.com/isexe/-/isexe-2.0.0.tgz#e8fbf374dc556ff8947a10dcb0572d633f2cfa10"
 
+js-string-escape@1.0.1:
+  version "1.0.1"
+  resolved "https://registry.yarnpkg.com/js-string-escape/-/js-string-escape-1.0.1.tgz#e2625badbc0d67c7533e9edc1068c587ae4137ef"
+
 js-yaml@^3.4.3:
   version "3.9.0"
   resolved "https://registry.yarnpkg.com/js-yaml/-/js-yaml-3.9.0.tgz#4ffbbf25c2ac963b8299dc74da7e3740de1c18ce"
@@ -439,6 +447,10 @@ number-is-nan@^1.0.0:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/number-is-nan/-/number-is-nan-1.0.1.tgz#097b602b53422a522c1afb8790318336941a011d"
 
+object-assign@4.1.0:
+  version "4.1.0"
+  resolved "https://registry.yarnpkg.com/object-assign/-/object-assign-4.1.0.tgz#7a3b3d0e98063d43f4c03f2e8ae6cd51a86883a0"
+
 object-assign@^4.0.1, object-assign@^4.1.0:
   version "4.1.1"
   resolved "https://registry.yarnpkg.com/object-assign/-/object-assign-4.1.1.tgz#2109adc7965887cfc05cbbd442cac8bfbb360863"
@@ -490,9 +502,12 @@ pg-connection-string@0.1.3:
   version "0.1.3"
   resolved "https://registry.yarnpkg.com/pg-connection-string/-/pg-connection-string-0.1.3.tgz#da1847b20940e42ee1492beaf65d49d91b245df7"
 
-pg-pool@2.*:
-  version "2.0.1"
-  resolved "https://registry.yarnpkg.com/pg-pool/-/pg-pool-2.0.1.tgz#8b12541df271b57f7020c50a3f5566471f82c77e"
+pg-pool@1.*:
+  version "1.8.0"
+  resolved "https://registry.yarnpkg.com/pg-pool/-/pg-pool-1.8.0.tgz#f7ec73824c37a03f076f51bfdf70e340147c4f37"
+  dependencies:
+    generic-pool "2.4.3"
+    object-assign "4.1.0"
 
 pg-types@1.*:
   version "1.12.0"
@@ -504,19 +519,20 @@ pg-types@1.*:
     postgres-date "~1.0.0"
     postgres-interval "^1.1.0"
 
-pg@^7.0.2:
-  version "7.0.2"
-  resolved "https://registry.yarnpkg.com/pg/-/pg-7.0.2.tgz#b9c2fe8168e7edfe9343aebe6fc48591e766ada7"
+pg@^6.1.0:
+  version "6.4.2"
+  resolved "https://registry.yarnpkg.com/pg/-/pg-6.4.2.tgz#c364011060eac7a507a2ae063eb857ece910e27f"
   dependencies:
     buffer-writer "1.0.1"
+    js-string-escape "1.0.1"
     packet-reader "0.3.1"
     pg-connection-string "0.1.3"
-    pg-pool "2.*"
+    pg-pool "1.*"
     pg-types "1.*"
-    pgpass "1.x"
+    pgpass "1.*"
     semver "4.3.2"
 
-pgpass@1.x:
+pgpass@1.*:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/pgpass/-/pgpass-1.0.2.tgz#2a7bb41b6065b67907e91da1b07c1847c877b306"
   dependencies: