SQLite: fix issues migrating and seeding

Related to issue #49 and issue #122 

* SQLite issues: added missing default values for columns custom_value1, custom_value2 in table invoices
* SQLite issues: corrected NOT NULL constraint violation when seeding gateways

I had these problems migrating and seeding the SQLite database:

	$php artisan migrate --seed
																							 
	  [Illuminate\Database\QueryException]                                                                      
	  SQLSTATE[HY000]: General error: 1 Cannot add a NOT NULL column with default value NULL (SQL: alter table  
	   "invoices" add column "custom_value1" float not null)
	   
and seeding the gateways:

	$ php artisan migrate --seed
	Migrated: 2013_11_05_180133_confide_setup_users_table
	Migrated: 2013_11_28_195703_setup_countries_table
	Migrated: 2014_02_13_151500_add_cascase_drops
	Migrated: 2014_02_19_151817_add_support_for_invoice_designs
	Migrated: 2014_03_03_155556_add_phone_to_account
	Migrated: 2014_03_19_201454_add_language_support
	Migrated: 2014_03_20_200300_create_payment_libraries
	Migrated: 2014_03_23_051736_enable_forcing_jspdf
	Migrated: 2014_03_25_102200_add_sort_and_recommended_to_gateways
	Migrated: 2014_04_03_191105_add_pro_plan
	Migrated: 2014_04_17_100523_add_remember_token
	Migrated: 2014_04_17_145108_add_custom_fields
	Migrated: 2014_04_23_170909_add_products_settings
	Migrated: 2014_04_29_174315_add_advanced_settings
	Migrated: 2014_05_17_175626_add_quotes
	Migrated: 2014_06_17_131940_add_accepted_credit_cards_to_account_gateways
	Migrated: 2014_07_13_142654_one_click_install
	Migrated: 2014_07_17_205900_support_hiding_quantity
	Migrated: 2014_07_24_171214_add_zapier_support
	Migrated: 2014_10_05_141856_track_last_seen_message
	Migrated: 2014_10_06_195330_add_invoice_design_table
	Migrated: 2014_10_13_054100_add_invoice_number_settings
	Running DatabaseSeeder
	Seeded: UserTableSeeder