travis: split up scripts into statements

Advantages:
- The build fails if a statement fails.
  Previously, errors in all but the last statement were silently
  ignored.
- The Travis log gives a fine-grained view of the output of each
  statement.

Add if statements because the '[…] &&' prefix results in YAML syntax
errors.

.travis.yml

@@ -3,19 +3,19 @@ language: minimal
 # broken:
 #   - PKG=electrs STABLE=0
 
+install:
     # Retry installing nix due to nondeterministic error
     #   Fatal error: glibc detected an invalid stdio handle
     # see:
     #   https://github.com/nh2/static-haskell-nix/pull/27#issuecomment-502652181
     #   https://github.com/nixos/nix/issues/2733
-install: |
-  (for i in {1..5}; do bash <(curl https://nixos.org/nix/install) && exit 0; done; exit 1)
-  . /home/travis/.nix-profile/etc/profile.d/nix.sh
-  nix-env -iA cachix -f https://cachix.org/api/v1/install
-  cachix use nix-bitcoin
-  [ $STABLE -eq 1 ] && export NIX_PATH="nixpkgs=$(nix eval --raw -f pkgs/nixpkgs-pinned.nix nixpkgs)"
-  [ $STABLE -eq 0 ] && export NIX_PATH="nixpkgs=$(nix eval --raw -f pkgs/nixpkgs-pinned.nix nixpkgs-unstable)"
-  VER="$(nix eval nixpkgs.lib.version)"
+  - (for i in {1..5}; do bash <(curl https://nixos.org/nix/install) && exit 0; done; exit 1)
+  - . /home/travis/.nix-profile/etc/profile.d/nix.sh
+  - nix-env -iA cachix -f https://cachix.org/api/v1/install
+  - cachix use nix-bitcoin
+  - if [[ $STABLE == 1 ]]; then export NIX_PATH="nixpkgs=$(nix eval --raw -f pkgs/nixpkgs-pinned.nix nixpkgs)"; fi
+  - if [[ $STABLE == 0 ]]; then export NIX_PATH="nixpkgs=$(nix eval --raw -f pkgs/nixpkgs-pinned.nix nixpkgs-unstable)"; fi
+  - VER="$(nix eval nixpkgs.lib.version)"
 env:
   matrix:
   - PKG=nodeinfo STABLE=1
@@ -29,6 +29,6 @@ env:
   - PKG=elementsd STABLE=0
   - PKG=electrs STABLE=1
   - PKG=liquid-swap STABLE=1
-script: |
-  printf '%s (%s)\n' "$NIX_PATH" "$VER"
-  nix-build -A $PKG
+script:
+  - printf '%s (%s)\n' "$NIX_PATH" "$VER"
+  - nix-build -A $PKG