workaround for TypeScript trailing comma bug

before this change, TypeScript compilation of the generated contract
wrapper was giving me the following errors:

$ abi-gen --abis 'build/contracts/*.json' --out build/types --template contract_templates/contract.handlebars --partials 'contract_templates/partials/*.handlebars'
Found 7 partial templates
Found 1 ABI files
Processing: Migrations...
Created: build/types/migrations.ts
$ tsc
build/types/migrations.ts(81,23): error TS1013: A rest parameter or binding pattern may not have a trailing comma.
build/types/migrations.ts(108,23): error TS1013: A rest parameter or binding pattern may not have a trailing comma.
build/types/migrations.ts(130,23): error TS1013: A rest parameter or binding pattern may not have a trailing comma.
build/types/migrations.ts(146,25): error TS1013: A rest parameter or binding pattern may not have a trailing comma.
build/types/migrations.ts(173,25): error TS1013: A rest parameter or binding pattern may not have a trailing comma.
build/types/migrations.ts(195,25): error TS1013: A rest parameter or binding pattern may not have a trailing comma.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Here is the generated code around the first error:

74:    public setCompleted = {
75:        async sendTransactionAsync(
76:            completed: BigNumber,
77:            txData: Partial<TxData> = {},
78:        ): Promise<string> {
79:            const self = this as any as MigrationsContract;
80:            const inputAbi = self._lookupAbi('setCompleted(uint256)').inputs;
81:            [completed,
82:    ] = BaseContract._formatABIDataItemList(inputAbi, [completed,
83:    ], BaseContract._bigNumberToString.bind(self));

All of the other errors are the same, a destructuring assignment with a
single element but with a trailing comma.

This is legal JavaScript but it is not allowed by the TypeScript
compiler, apparently per the bug described at
microsoft/TypeScript#24628 .

While awaiting the 3.0 version of TypeScript, it's a simple enough
change to have the template not append a trailing comma.

packages/contract_templates/partials/params.handlebars

@@ -1,3 +1,3 @@
 {{#each inputs}}
-{{name}},
+{{name}}{{#if @last}}{{else}},{{/if}}
 {{/each}}