Check PG version / support for PG v12

[adiktofsugar]

Hey, I just tried to use this and got this error:

{
  method: 'raw',
  sql: 'select\n' +
    '    c.conname as "indexName",\n' +
    `    array_to_string(array_agg(a.attname), ',') as "columnNames",\n` +
    '    c.consrc as "expression"\n' +
    '  from\n' +
    '    pg_class t,\n' +
    '    pg_constraint c,\n' +
    '    pg_namespace n,\n' +
    '    pg_attribute a\n' +
    '  where\n' +
    '    c.conrelid = t.oid\n' +
    '    and n.oid = c.connamespace\n' +
    '    and a.attrelid = t.oid\n' +
    '    and a.attnum = ANY(c.conkey)\n' +
    "    and c.contype = 'c'\n" +
    '    and t.relname = ?\n' +
    '    and n.nspname = ?\n' +
    '  group by\n' +
    '    t.relname,\n' +
    '    c.conname,\n' +
    '    c.consrc;',
  bindings: [ 'drink', 'public' ],
  options: {},
  __knexQueryUid: '7945a2dd-19ff-40b0-9ed5-e8d00d48152f'
}
Error migrating: error: column c.consrc does not exist
    at Parser.parseErrorMessage (/my/cool/project/node_modules/pg-protocol/src/parser.ts:325:11)
    at Parser.handlePacket (/my/cool/project/node_modules/pg-protocol/src/parser.ts:154:21)
    at Parser.parse (/my/cool/project/node_modules/pg-protocol/src/parser.ts:106:30)
    at Socket.<anonymous> (/my/cool/project/node_modules/pg-protocol/src/index.ts:7:48)
    at Socket.emit (events.js:310:20)
    at addChunk (_stream_readable.js:286:12)
    at readableAddChunk (_stream_readable.js:268:9)
    at Socket.Readable.push (_stream_readable.js:209:10)
    at TCP.onStreamRead (internal/stream_base_commons.js:186:23) {
  length: 187,
  severity: 'ERROR',
  code: '42703',
  detail: undefined,
  hint: 'Perhaps you meant to reference the column "c.conkey" or the column "c.conbin".',
  position: '107',
  internalPosition: undefined,
  internalQuery: undefined,
  where: undefined,
  schema: undefined,
  table: undefined,
  column: undefined,
  dataType: undefined,
  constraint: undefined,
  file: 'parse_relation.c',
  line: '3377',
  routine: 'errorMissingColumn'
}

After a little research I noticed this seems to be a problem with the upgrade of postgres v11 -> v12.
Preferably you'd support v12, but it might be better to just check the version to give a more descriptive error message.