Make COIN env variable mandatory.

- make a clean split between the  Cash and Segwit flavours of bitcoin
  by giving them their own COIN names.  They can then both have a NET
  of mainnet.
- The previous Bitcoin COIN names no longer exist, and the env var is
  now mandatory, so everyone will need to set COIN and NET appropriately
  for their flavour of bitcoin and mainnet or testnet.

ENVIRONMENT.rst

@@ -13,6 +13,11 @@ Required
 
 These environment variables are always required:
 
+* **COIN**
+
+  Must be a *NAME* from one of the **Coin** classes in
+  `lib/coins.py`_.
+
 * **DB_DIRECTORY**
 
   The path to the database directory.  Relative paths should be
@@ -53,11 +58,6 @@ Miscellaneous
 
 These environment variables are optional:
 
-* **COIN**
-
-  Must be a *NAME* from one of the **Coin** classes in
-  `lib/coins.py`_.  Defaults to `Bitcoin`.
-
 * **NET**
 
   Must be a *NET* from one of the **Coin** classes in `lib/coins.py`_.